(1) Field of the Invention
The present invention relates generally to a process producing mixed esters of fatty acids as biofuels particularly for use in a compression ignition (CI) (particularly diesel) engine.
(2) Description of Related Art
Biodiesel has been considered as an alternative to petroleum based diesel for many years. Biodiesel is a general term referring to a fuel comprising methyl-esters of long chain fatty acids derived typically from vegetable oils or animal fats. It can be used per se as fuel, or as an additive in a blend with petroleum-based diesel fuel.
The biodiesel industry in the U.S. currently suffers from a lack of consumer confidence in the quality of the biofuel. The uncertainty in fuel quality stems from lack of experience on the part of producers in being consistent in their production methods, and the all-too-often absence of careful fuel analysis to ensure that quality standards such as ASTM specifications are achieved. Although several measures of biodiesel quality can be used, the limiting factor is often the cold-weather performance properties of biodiesel, manifested as a cloud point and pour point temperature that is too high for the climate. A high cloud point temperature in biodiesel is typically observed because of (1) glyceride impurities present and (2) the presence of only methyl esters of unsaturated and saturated fatty acids. If residual glyceride impurities are not removed from the finished product, the biodiesel forms crystals at low temperature and those crystals plug fuel filters and injectors. Even if the impurities are removed, the saturated fatty acid methyl esters crystallize at some point as temperature is reduced, thus leading to solids formation. Thus, auto and diesel engine manufacturers at present will only warrant biofuel compositions up to a biofuel content of B5 (5% biodiesel/95% petroleum). Ideally, a high quality biofuel for the North American climate would contain at least B20 (20% biodiesel, 80% petroleum).
Biodiesel offers several advantages. In particular, when compared to petroleum diesel, biodiesel provides similar fuel economy, horsepower and torque while providing superior lubricity. Moreover, biodiesel provides a substantial reduction of emission of unburned hydrocarbons, carbon monoxide, and particulate matter. Typically, it is free of sulfur and aromatics which are major pollutants. Accordingly, biodiesel is considered a renewable, non-toxic and biodegradable fuel alternative or additive.
Previous methods associated with producing mixed esters of fatty acids include reaction of (1) a mixture of different triglycerides with methanol; or (2) an oil (triglycerides) with a mixture of different alcohols, namely ethanol, methanol, n-butanol and n-propanol, in the presence of a base catalyst chosen from sodium hydroxide, potassium hydroxide and sodium methylate. In presence of a base catalyst (sodium or potassium hydroxides), the rate and extent of ester formation are directly proportional to the formation of sodium or potassium alkoxide from the alcohol in situ in the reaction mixture.
Upon completion of the reaction, ideally two distinct phases, 1) glycerol (a trihydroxy alcohol) and 2) esters of fatty acids, are observed. The following limitations are typically associated with this process: 1) the transesterification reaction proceeds smoothly only when methanol is used as an alcohol and fatty acid methyl ester (FAME) is synthesized, and the reaction is adversely affected when higher alcohols such as ethanol, n-propanol and n-butanol are used; 2) use of higher alcohols also creates a problem of readily separating glycerol from the alkyl esters of fatty acids, which requires additional processing steps including alcohol separation from the reaction mixture and dilute acid wash to facilitate glycerol phase separation from fatty acid ester; and 3) use of base catalyst increases the prospect of soap formation (saponification of the fatty acids) which is quite detrimental to overall process and its economy. It is a further disadvantage of existing processes that the base catalysts used in the reaction system are not re-usable, thereby generating a considerably significant quantity of salt waste.
U.S. Patent Application No. 2007/0056213 to French et al. describes a method which includes operating a two-stroke engine with a lubricating fuel. A fuel/lubricant formulation is disclosed for the operation of the two-stroke engines with improved emissions and performance. The lubricating fuel includes at least one fuel selected from the group including C1-6 alcohol, gasoline, ether, ketone, nitromethane, and a mixture thereof, and at least one lubricant selected from the group including biodiesel, lipid fatty acid alkyl ester, fatty acid and a mixture thereof. Diesel fuels are not described.
Patent application WO 2006/107407 to Clements describes processes and systems for producing biodiesel or fatty acid esters from multiple triglyceride feedstocks using a two step reaction with an alcohol and acid catalyst and then an alkaline catalyst. The first step forms an acid alcohol layer and an ester-triglyceride layer. The second step reacts the ester triglyceride layer with a base to form the fatty acid esters.
Patent application WO 2006/128881 to Despeghel describes alkyl-ester compositions derived from rapeseed and sunflower, in particular from Brassica napus and Helianthus annuus using an acid catalyst in a batch process. Despeghel further discloses a process for preparation of the alkyl-esters. The alkyl-ester compositions can be used in diesel engines in its pure form or blended with another composition such as fuel.
U.S. Pat. No. 6,468,319 to Yeh et al. describes various esters used in diesel fuel to reduce emissions. U.S. Pat. No. 5,268,008 to Kanne describes esters used in diesel fuels to reduce emissions.
While the related art teach processes for generating biodiesel, there still exists a need for improved processes and compositions for generating biodiesel.